The present invention relates to a transmitting method and a transmitting system for data transmission between apparatuses connected through an IEEE 1394 system bus, for example, and a transmitter to which the transmitting method may be applied.
AV apparatuses are currently capable of mutually transmitting information through a network using an IEEE 1394 system serial data bus. Data transmission is carried out employing such a bus in an isochronous transfer mode. This transfer mode is used for transmitting, in real time, animation data, audio data and the like which comprise a comparatively large amount of data. An asynchronous transfer mode may also be used for transmitting a static image, text data, control command and the like, which have relatively small amounts of data. A special frequency band is used for the transmission in each transfer mode. Therefore, a transmitting operation can be carried out in both modes simultaneously on one bus.
When a relatively large amount of data requiring a comparatively large capacity is to be transmitted in the isochronous transfer mode between a plurality of apparatuses connected through such a bus, it is possible to control the transmission speed of the data from a transmitting source under the control of a receiving apparatus. More specifically, as is shown in FIG. 13 for example, digital data are synchronously transmitted from a transmitter 101 to a receiver 102. As long as the clock rate in transmitter 101 and the clock rate in receiver 102 are coincident with each other, there is no synchronization problem. However, the clock rate for each apparatus usually has some errors, and thus they are usually not precisely coincident. Therefore, it is necessary to carry out some processing to absorb errors between the clock rates. A buffer 103 is provided with receiver 102. A control command is sent in the asynchronous transfer mode from receiver 102 to control or adjust the transmission rate of data from transmitter 101 such that the buffer 103 neither underflows nor overflows.
The data transmitted from transmitter 101 can therefore be correctly received through receiver 102. An AV/C command set may be employed for controlling a transmission rate for transmission from receiver 102 to transmitter 101. The AV/C command set includes commands for controlling an apparatus connected through an IEEE 1394 serial bus, for example. The AV/C command set is standardized with 1394 TA (Trade Association), and the details of the AV/C command are disclosed on the 1394 trade association home page, for example.
In order for receiver 102 to control the transmission rate of data from transmitter 101, it is necessary that each transmitter 101 corresponds to one and only one receiver 102. Only one receiver 102 can transmit a rate control command to a particular transmitter 101. If more than one receiver is connected to the bus when a rate control command sent from any of he receivers connected to the bus, it is also accepted by transmitter 101, an appropriate transmission rate might be disturbed in response to a control command sent from any one or more of the plurality of receivers. Only if transmitter 101 and receiver 102 are in a one to one correspondence can such control can be carried out.
Furthermore, if receiver 102 stops controlling the transmission rate for some reason, such as because of an error, there is a possibility that transmission rate control of transmitter 101 cannot be carried out by the corresponding receiver 102. In such a case, the transmission should be stopped at once so that the rate control can be carried out by other receivers.